The present invention relates to mountings for rotatory parts, and more particularly to supports for the shafts, spindles or analogous rotary elements (hereinafter called shafts for short) which carry and transmit torque to or receive torque from tools in material removing machines.
It is known to mount two spaced-apart portions of a shaft for a grinding or dressing tool in discrete antifriction bearings which, in turn, are mounted in a housing or frame in such a way that they allow for thermally induced changes in the length of the shaft. Such mounting is advantageous in connection with rotatory wheel-shaped dressing tools which are used to treat the working surfaces of grinding wheels in surface grinding or other types of grinding machines.
The grinding wheel or wheels of a grinding machine must be dressed, either continuously or at certain intervals, in order to ensure that the profile of the working surface of each grinding wheel matches an optimum outline which ensures adequate treatment of workpieces. Continuous or frequent dressing of the working surfaces of grinding wheels is especially desirable in modern high-precision grinding machines which must treat selected surfaces of workpieces with an extremely high degree of accuracy. In order to allow for thermally induced changes in the axial length of the shaft which carries the dressing tool, such shaft is normally mounted in a fixedly installed bearing (i.e., a bearing which is held against axial movement) and in a loosely mounted bearing which can move axially toward or away from the fixedly installed bearing. This enables the shaft for the dressing tool (which is mounted on the shaft between the two bearings) to move the loosely installed bearing axially toward or away from the fixedly installed bearing. As a rule, the fixedly installed bearing is a needle bearing. This is considered desirable and advantageous because a needle bearing allows for uninterrupted changes in the length of the shaft so that such changes can be readily compensated for by an experienced attendant who is in charge of supervising and regulating the dressing operation or by an automatic compensating system if the dressing tool is installed or put to use in a numerically controlled or otherwise programmed grinding machine.
However, needle bearings cannot be used when the dressing tool must treat the working surfaces of grinding wheels in modern heavy-duty grinding machines. Needle bearings cannot stand pronounced stresses which arise in such material removing machines and, therefore, they must be replaced with sturdier and more stress-resistant roller bearings. On the other hand, the bearings which can stand much higher stresses than a needle bearing exhibit the drawback that they do not allow for uninterrupted shortening or lengthening of the shaft for a grinding wheel, dressing tool or an analogous tool in a grinding or another material removing machine. On the contrary, rather than allowing for uninterrupted changes in the axial length of the shaft (i.e., for a change whenever the need arises in view of the changing temperature of the shaft), such bearings merely allow for jerky or sporadic changes in the axial length of the shaft (this is known as the stick-slip effect). Thus, the loose bearing can move toward or away from the fixedly installed bearing only when the stresses which develop as a result of the tendency of the shaft to change its length reach a predetermined value. Each such rise of stresses is followed by an abrupt shifting of the loose bearing in a direction toward or away from the fixedly installed bearing. The just described mode of changing the length of the shaft prevents even a highly skilled attendant from adequately compensating for changes in the axial length of the shaft (i.e., from ensuring adequate dressing of the working surface of the grinding wheel) and, in fact, even a highly sophisticated automatic compensating system of a numerically controlled or other ultramodern grinding machine is incapable of coping with this problem.
German Offenlegungsschrift No. 27 47 976 of Voumard discloses a support for the shaft or spindle of a grinding wheel which employs a membrane serving to connect a stationary housing part (stator) with an axially movable ball bearing for one end portion of the shaft. Such membrane allows for shortening or lengthening of the shaft in response to changes in temperature. However, and since the central portion of the membrane is connected to the loose bearing and the marginal portion of the membrane is connected to the stator all the way around the bearing, all changes in the length of the shaft entail the development of very pronounced tensional stresses in addition to equally pronounced bending or flexing stresses. The tensional stresses are transmitted to the stator as well as to the loose bearing with attendant highly pronounced wear upon and stray movements of the shaft and the material removing tool thereon. Proposals to reduce such tensional stresses involve the utilization of a specially designed membrane with marginal cutouts to enhance the flexibility of the respective sections. This contributes significantly to the initial cost of the support for the shaft as well as to the cost of maintaining the support in proper condition and of monitoring the distribution of stresses in the membrane and/or in the parts which are connected to the membrane.
An additional drawback of a membrane is that its rigidity changes unpredictably in response to relatively small deformation so that the resistance of the membrane to axial and/or radial movements of the loose bearing is quite erratic as soon as the membrane undergoes a relatively small amount of deformation. This prevents the attendants from adequately compensating for changes in the axial position of the dressing tool relative to the grinding wheel when the length of the shaft for the dressing tool changes. The same holds true for the equipment which is used in a modern grinding machine to effect an automatic compensation for axial shifting of the dressing tool.